Power generation from low-temperature heat

ABSTRACT

The invention relates to a method for converting heat energy into mechanical energy by means of a Rankine cycle. In the Rankine cycle the circulating working fluid is pumped to a pressure above its critical pressure prior to heat exchange with an external medium. During the heat exchange with the external medium, the working fluid is heated to a temperature above its critical temperature and sufficiently high for the working fluid to expand without partial condensation. The working fluid is then expanded and condensed. The maximum pressure of the working fluid is controlled by means of an expander controllable with regard to the mass flow rate of the working fluid and/or a pump controllable with regard to the mass flow rate of the working fluid.

SUMMARY OF THE INVENTION

The invention relates to a method for converting heat energy into mechanical energy by means of a Rankine cycle. In particular, the invention relates to a method wherein: a working fluid circulating in a Rankine cycle is pumped to a pressure above its critical pressure prior to heat exchange with an external medium; and the working fluid is then heated during heat exchange with the external medium to a temperature above its critical temperature, the temperature being at least sufficiently high for the working fluid to expand without partial condensation. The working fluid is then expanded; and the expanded working fluid is condensed.

The conversion of heat energy into mechanical or electrical energy by means of a Rankine cycle has long been known and is mainly used, for example, in thermal power stations of all types. However, if the maximum available temperature level is limited, the use of water or water vapor as the working fluid circulating in the Rankine cycle results in uneconomically large plants due to low vapor densities. Therefore, in such cases other working fluids, such as ammonia for example, are used, which allow an optimum operating density for the available heat source.

A method of the above type for converting heat energy into mechanical energy by means of a Rankine cycle is described, for example, in U.S. Pat. No. 6,751,959. This process is explained in simplified form with reference to FIG. 1.

The working fluid, preferably water or ammonia, is brought to a pressure beyond critical pressure by means of the pump P10 and fed via line 10 to the heat exchanger E10. An external medium, for example hot water, is fed to the heat exchanger via line A. The heat energy of the external medium is converted into mechanical or electrical energy by means of the Rankine cycle. This external medium, cooled in the heat exchanger E10 against the working fluid, is then drawn off via line B.

The heat exchanger E10 and the composition of the working fluid should be respectively designed or selected such that the working fluid 10 is heated in the heat exchanger E10 up to a temperature above its critical temperature. By means of this procedure, the sensible heat fed to the heat exchanger E10 by the medium A may be particularly well utilized. If the temperature of the working fluid 11 downstream of the heat exchanger E10 is sufficiently far, typically at least 30 K, above its critical temperature, the expander X10 may be operated in the gas phase and undesired partial condensation of the working fluid in the expander X10 may thus be avoided. The expander X10 is connected to a generator G.

The valve V10 serves to keep the pressure of the working fluid in the heat exchanger E10 above critical pressure. The expanded working fluid 12 is not only completely condensed in the heat exchanger E20 but rather is additionally subcooled and then fed to the storage or surge tank D10. From there it passes via line 13 back to the pump P10.

The procedure described in U.S. Pat. No. 6,751,959 explicitly omits heat supply or heat removal outside the heat exchangers E10 and E20. However, the Rankine cycle design described in U.S. Pat. No. 6,751,959 makes it difficult to achieve optimum operation of the expander X10, since the inlet pressure of this expander is not controlled, and additionally reduces the efficiency of the method.

An aspect of the present invention, therefore, is to provide a method of the above type for converting heat energy into mechanical energy by means of a Rankine cycle, which method avoids the above-stated disadvantages, and in particular makes possible the achievement of higher efficiency.

Upon further study of the specification and appended claims, other aspects and advantages of the invention will become apparent.

To achieve these aspects, a method for converting heat energy into mechanical energy by means of a Rankine cycle is proposed wherein the maximum pressure of the working fluid is controlled by means of an expander controllable with regard to the mass flow rate of the working fluid and/or a pump controllable with regard to the mass flow rate of the working fluid.

In contrast to the above-described procedure belonging to the prior art, the maximum pressure of the working fluid is not controlled by means of a valve, but rather by means of an expander controllable with regard to the mass flow rate of the working fluid and/or a pump controllable with regard to the mass flow rate of the working fluid.

The expander controllable with regard to the mass flow rate of the working fluid preferably comprises an adjustable inlet guide vane (see, e.g., US 2009/0238681), which preferably comprises a nozzle ring at the inlet of the expander.

By means of the pump controllable with regard to the mass flow rate of the working fluid, the desired operating pressure of the working fluid may be established at the inlet of the heat exchanger, in which heat exchange takes place between the working fluid and the external medium.

Through the procedure according to the invention, the inlet state of the expander, which is needed for smooth operation of the Rankine refrigeration cycle, is stabilized.

In accordance with one advantageous configuration of the method according to the invention, the working fluid is additionally heated in particular during the start-up procedure and/or part load operation. This configuration of the method according to the invention requires an additional heat exchanger and an (additional) media stream, which is able to provide a sufficiently high level of heat. This additional heating of the working fluid advantageously ensures that the temperature of the working fluid is at least 30° C., preferably 40 to 60° C., above the critical temperature even during the start-up procedure and/or part load operation. Thus, this configuration makes it possible to keep the inlet temperature of the expander substantially constant even during the start-up procedure and/or part load operation.

Advantageously, the expanded working fluid is used to preheat the pumped working fluid, before the latter undergoes heat exchange with the external medium. This configuration of the method according to the invention is especially meaningful when the outlet temperature of the working fluid on leaving the expander is higher than the condensation temperature in the heat exchanger downstream of the expander. In this case the heat from the temperature interval between the outlet temperature and the condensation temperature may be used to preheat the working fluid.

As a further development of the method according to the invention, it is proposed for the expanded working fluid to be condensed, but not subcooled. Subcooling of the working fluid may in particular be omitted when the storage tank to be provided is positioned sufficiently high above the pump to prevent undesired cavitation in the pump. The latter two configurations of the method according to the invention lead to an improvement in the efficiency of the method according to the invention.

According to a further advantageous configuration of the method according to the invention, the pressure of the working fluid at the inlet of the expander is at least 30%, preferably between 40 and 50%, above the critical pressure of the working fluid. As a result of this minimum difference from the respective critical pressure of the working fluid, undesirably severe variations in material properties, such as for example density and viscosity, may be reliably prevented; these variations may occur in the vicinity of the critical point in the event of slight changes in pressure and/or temperature.

As a further development of the method according to the invention, it is proposed that propane, propylene or any desired mixture of propane and propylene circulates in the Rankine cycle as the working fluid. This configuration is particularly advantageous when the temperature of the external medium A amounts to between 120 and 200° C., preferably between 130 and 160° C.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the invention are to be explained with the following description of the Figures of exemplary embodiments on the basis of the figures, in which:

FIG. 1 shows Rankine cycle system in accordance with the prior art; and

FIG. 2 illustrates an exemplary embodiment of a Rankine cycle system according to the invention.

The working fluid circulating in the Rankine cycle is brought to the desired operating pressure by means of the pump P1 and preheated in the heat exchanger E3 against the expanded working fluid 5. According to the invention, a pump P1 is provided which may be controlled with regard to the mass flow rate of the working fluid 1. The working fluid is pumped by means of the pump P1 to a pressure such that it may be ensured that the pressure of the heated working fluid 4 at the inlet to the expander X1 is at least 30%, preferably between 40 and 50%, above the critical pressure of the working fluid.

The working fluid is preheated in the heat exchanger E3 and fed via line 2 to the heat exchanger E1, to which an external medium, for example hot water, is fed via line A. This external medium is cooled in the heat exchanger E1 against the working fluid and drawn off from the heat exchanger E1 via line B. The working fluid drawn off from the heat exchanger E1 via line 3 is preferably heated to a temperature at least 30 K, preferably 40 to 60 K, above its critical temperature.

The heat exchanger E4 serves to heat the working fluid by a suitable (additional) external medium, preferably during the start-up procedure and/or during part load operation. When the Rankine cycle is in normal operation, this heat exchanger is not required. The working fluid is fed to the expander X1 via line 4. According to the invention, the expander X1 is an expander controllable with regard to the mass flow rate of the working fluid. To this end, the expander X1 preferably comprises an adjustable inlet guide vane Y, which preferably is a nozzle ring at the inlet of the expander.

The expanded working fluid is fed via line 5 to the heat exchanger E3. The preheating of the pumped working fluid 1 carried out in the heat exchanger E3 is particularly convenient when the outlet temperature of the working fluid 5 on leaving the expander X1 is higher than the condensation temperature in the heat exchanger E2 downstream of the expander X1. In this case the heat from the temperature interval between the outlet temperature and the condensation temperature in the heat exchanger E3 may be used to preheat the working fluid 1.

The expanded working fluid is fed to the heat exchanger E2 via line 6 and is condensed and subcooled therein against a suitable external medium. The subcooled working fluid is then fed via line 7 to the storage or surge tank D1. From there it passes via line 8 back to the pump P1. Subcooling of the working fluid 6 in the heat exchanger E2 may be dispensed with if the storage tank D1 is positioned sufficiently high above the pump P1 to prevent undesired cavitation in the pump P1.

If the temperature of the external medium A fed to the heat exchanger E1 is between 120 and 200° C., preferably between 130 and 160° C., propane, propylene or any desired mixture of propane and propylene is preferably used as the working fluid.

The method according to the invention for converting heat energy into mechanical energy allows stabilization of the inlet state of the expander X1, which results in improved operation of the Rankine refrigeration cycle. The method according to the invention additionally exhibits greater efficiency than the above-described, prior art method.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 10 2011 109 777.9, filed 9 Aug. 2011, are incorporated by reference herein. 

1. A method for converting heat energy into mechanical energy by means of a Rankine cycle, said method comprising: pumping a working fluid circulating in a Rankine cycle to a pressure above the critical pressure of said working fluid prior conducting heat exchange between said working fluid and an external medium and heating said working fluid during said heat exchange with said external medium to a temperature above the critical pressure of said working fluid, said temperature being at least sufficiently high for the working fluid to expand without partial condensation, expanding said working fluid in, and condensing the expanded working fluid, wherein the maximum pressure of said working fluid is controlled by means of an expander (X1) controllable with regard to the mass flow rate of the working fluid (5) and/or a pump (P1) controllable with regard to the mass flow rate of the working fluid (1).
 2. The method according to claim 1, wherein the maximum pressure of said working fluid is controlled by means of an expander (X1) controllable with regard to the mass flow rate of the working fluid (5).
 3. The method according to claim 1, wherein the maximum pressure of said working fluid is controlled by means of a pump (P1) controllable with regard to the mass flow rate of the working fluid (1).
 4. A method according to claim 1, wherein expander controllable with regard to the mass flow rate of the working fluid comprises an adjustable inlet guide vane.
 5. A method according to claim 4, wherein the adjustable inlet guide vane comprises a nozzle ring at the inlet of the expander.
 6. The method according to claim 1, wherein said working fluid (3) is further heated by an additional heat exchange during start-up and/or during part load operation (E4).
 7. The method according to claim 1, wherein expanded working fluid (5) is used to preheat (E3) said working fluid (1).
 8. The method according to claim 6, wherein expanded working fluid (5) is used to preheat (E3) said working fluid (1) in said additional heat exchange.
 9. The method according to claim 1, wherein expanded working fluid (5) is condensed (E2), but not subcooled.
 10. The method according to claim 1, wherein the pressure of said working fluid (4) at the inlet of said expander (X1) is at least 30%, above the critical pressure of said working fluid.
 11. The method according to claim 10, wherein the pressure of said working fluid (4) at the inlet of said expander (X1) is 40 and 50% above the critical pressure of said working fluid.
 12. The method according to claim 1, wherein said working fluid is propane, propylene or a mixture of propane and propylene.
 13. The method according to claim 1, wherein the temperature of said external medium is 120-200° C.
 14. The method according to claim 13, wherein the temperature of said external medium is 130-160° C.
 15. A method according to claim 1, wherein said working fluid is water or ammonia. 